The present invention generally relates to an air-fuel ratio detecting for detecting the air-fuel ratio of an air-fuel mixture supplied to a combustion device, and particularly to an air-fuel ratio detecting device provided with a detecting element section in which an oxygen sensing element and an oxygen pumping element are disposed in opposition to each other with a gap therebetween, each of the two elements being made of an oxygen-ion-conductive solid electrolyte provided with porous electrodes on opposite surfaces thereof, whereby the air-fuel mixture can be accurately detected over the entire operating range from rich to lean.
Conventionally, in a combustion device such as an engine or the like, feedback control of the air-fuel mixture has been carried out by detecting the concentration of oxygen in the exhaust gas and controlling the intake air-fuel ratio so as to maintain it near a theoretical (stoichiometric) value, thereby to reduce the fuel consumption rate and maintain a low level of unwanted emissions. For the oxygen sensor used in the system, generally an oxygen sensor is employed of a type in which the output voltage changes in a switching mode at the theoretical air-fuel ratio. An example of such a sensor is an oxygen sensor in which an ion-conductive solid electrolyte is coated with a porous electrode layer, and the air-fuel ratio in the vicinity of the theoretical value is detected on the basis of variations in an electromotive force generated between the electrode layers due to a difference in oxygen partial pressure between the exhaust gas and the atmosphere.
Recently, it has been considered desirable that the air-fuel ratio of the air-fuel mixture not just be maintained at a value in the vicinity of the theoretical air-fuel ratio, but controlled to a desired value in accordance with the present running state of the engine so as to further improve fuel economy and reduce emissions as well as improve the overall running performance of the engine. In the conventional oxygen sensor described above, however, only the theoretical air fuel ratio of the air-fuel mixture could be unambiguously detected, and therefore it was impossible to control the air-fuel mixture to a desired air-fuel ratio.
In order to realize feedback control of the air-fuel ratio as described above, there has been proposed an oxygen sensor in which two elements, each made of a plate-like oxygen-ion-conductive solid electrolyte provided with electrode layers on its opposite surfaces, are mounted parallel to one another with a gap therebetween communicating with the exhaust gas. One of the elements is used as an oxygen pumping element and the other as an oxygen sensing element, the latter producing an output in accordance with the difference in oxygen density between the atmosphere and the gap. This sensor produces an accurate output in the lean region of the air-fuel mixture where there is residual oxygen in the exhaust gas. However, in the rich region where there is no residual oxygen, due to reaction with CO, CO.sub.2, H.sub.2 O, and the like, the output of the sensor was not accurate.